1. Field of the Invention
The present invention relates to apparatus for the measurement of the mechanical output of induction motors and, more particularly, to circuit means for providing a quick and easy measurement of the output power of an induction motor.
2. Description of the Prior Art
It is widely known that the mechanical output of rotary machines is given by the product of torque and rotary speed. Both torque and rotary speed are measurable by commercially available instruments. However, before one can measure torque using available instruments, the installation of a load cell is usually required and the capacity and sensitivity of such load cell must be decided upon depending upon the object of the measurements. The labor required to install the load cell is undesirable when one wants quick and easy measurements and the selection of a load cell limits the versatility of the measuring instrument.
It is also known that a watt-hour meter is capable of measuring the electric power consumption of an induction motor independently of the power factor. This being the case, the difference of the readings obtained for a motor operating with and without a load corresponds to the mechanical output of the motor. However, with this type of measurement, the selection of the watt-hour meter depending upon the consumption of the motor and wiring of the meter to the motor are required. These requirements are similarly undesirable as was the case with the requirements of torque measurements. In addition, a watt-hour meter does not indicate the instantaneous consumption of electric power. Rather, it only indicates accumulative consumption. This is undesirable when the measurement is done to trace varying load conditions of the motor.
By using three ammeters, one can obtain an instantaneous reading of power consumption. However, it is virtually impossible for the average person to read three ammeters at a time, especially when loading conditions are quickly changing.
For most induction motors manufactured and sold commercially, the relationship between torque and current and/or torque and rotary speed are known. By using a tachometer or an ammeter, the output of the motor is measurable, although calculations based upon the above-mentioned relationship are required. Between measurements made by an ammeter and by a tachometer, the latter is preferred because no wiring and no selection of capacity are required.
Among the known measuring methods mentioned hereinabove, the method which employs a tachometer seems to be the best as far as simplicity is concerned, but a serious problem still exists, as will now appear. Specifically, FIG. 1 shows the relationship between mechanical output (torque) and the rotary speed of a typical induction motor, where Tm is rated maximum output, H is the rotary speed at rated maximum output, and S is synchronous speed. Synchronous speed (S) is calculated from the line frequency (f) and the number of poles of the motor (P) by the following equation, which is independent of the output capacity: EQU S=120 f/P. (1)
The slip of an induction motor is proportional to the difference between the synchronous speed and the actual speed as follows: EQU Sl=(S-H)/S.times.100%, (2)
where Sl is the slip.
For most induction motors, slip as rated maximum ranges from 0.3 to 5%. Due to the very small value of slip, only a very narrow portion of the scale of a tachometer is useful as an effective reading range and precision of the scale reading is very poor. For example, if one is to determine the output of a 4-pole motor, the slip of which is 5% and which is operated at a line frequency of 50 Hz, the rotary speed of the motor varies from 1500 to 1425 rmp as the load changes from 0-100% of the rated maximum. Assuming that the tachometer has a scale of 0 to 1500 rmp divided by 100, misreading of one division causes only a 1% error when it is used as a tachometer but when the tachometer is used as an output meter, the same misreading causes a 20% error.
The direct use of a tachometer as an output meter has another disadvantage. That is, the higher the output of the motor, the lower the indication on the scale. Such an output indication is against human nature.